The present invention relates to a process of making optical components in gel-silica, and, in particular, to an improved process of making channel waveguides therein.
Optical channel waveguides are fundamental devices in optical signal processing as they interface fiber optics with electro-optic circuits which convert the optical information into electronic signals. The simplest method available today for the production of such devices is ion exchange at the surface of silica-based glasses. In this method, high mobility silica network modifiers, such as Na.sub.2 O, are exchanged with monovalent ions, e.g., alkali, thallium and silver ions. Ion size and polarization effects lead to a localized increase in the refractive index of the glass. This procedure usually required masking of the substrate by photolithography, followed by immersion of the masked substrate in a controlled environment containing the dopant. Drawbacks in this procedure are the need of chemical manipulation with its respective environmental implications and the poor flexibility of the photolithographic processes, as new masks have to be produced for each new waveguide designed.
Laser direct writing is a substitute for the photolithographic based processes, offering lower overall process costs, higher circuit density and improved design flexibility. This statement only holds as long as substrates are suitable for the laser irradiation and can be provided for the desired application. For optical communications such substrates are the Type VI porous gel-silica optics which can be densified by the heat generated in the interaction with a CO.sub.2 laser beam lasing at 10.6 .mu.m.
Gel-silica monoliths with 14 .ANG. pore radius are today the only commercially produced product and have excellent optical properties. Previous studies used substrates with 14 .ANG. pores and with an initial density of 2.0 g/cc since lower densities of the 14 .ANG. material have a strong tendency to foam and/or crack under CO.sub.2 laser scans.
Preliminary studies showed that laser densified tracks could be made in gels with a pore radius of 45 .ANG. and a density of 1.1 g/cc. Other densities of these gels have not been used for laser densification because the 45 .ANG. material densifies rapidly at 1150.degree. C. Although their as-cast surface quality may cause too much scatter for an optical communications device, the 45 .ANG. gels are a useful system for laser densification, as they only tend to foam under very extreme conditions, and yield a large value of index gradient.
Previous studies showed that laser densification could be used to produce gradient refractive index (GRIN) optical devices on Type VI gel-silica substrates. In order to achieve optically transparent optical components it was necessary to control processing variables of substrate density, relative humidity, laser power, and distance of the objective lens to the substrate.